1. Field of the Invention
This invention relates to dielectric paste used in a ceramic wiring substrate for mounting semiconductor ICs, chip components and the like, and a method of manufacturing the dielectric paste.
2. Description of the Prior Art
Recently, with the development of the semiconductor industry and progress of the general electronic component industry, ICs, LSIs and small components have come to be widely used. In such situation, ceramic multilayer wiring substrates are attracting attention because ICs and LSIs can be directly mounted thereon, and wiring density can be extremely increased.
Conventional methods of manufacturing the ceramic multilayer wiring substrates are roughly classified into two-the thick film printing method and the green sheet laminating method. In the thick film printing method, thick film paste of conductors or insulators is laminated on a sintered ceramic substrate by repeating printing and firing alternately. The advantages of this method are that the thick film paste is readily available, and that the process itself is simple. In this method, however, since glass paste is used as the thick film paste of insulators, fluidity due to softening of the glass occurs when firing, so that the desired dimensions of the finely printed conductor pattern is not maintained. Accordingly, it is required to fire every time after printing each conductor layer and insulator layer, which increases, the lead time, resulting in an increase of cost. Consequently this method it is not suited to produce a multilayer wiring substrate with a great number of laminations. Besides, the head conductivity and mechanical strength of glass are inferior to those of alumina. On the other hand, in the green sheet laminating method, a conductor paste is printed on insulating green sheets before firing, and these green sheets are overlaid and laminated by heat and pressure, and thereafter fired. This method is preferably used in applications with a great number of laminations. The advantages of this method are that the lead time is short because of using only one firing process, and the density of the insulation layers are increased easily because of the lamination by pressure. But it had its own shortcomings. That is, since alumina was conventionally used as the insulating material, the firing temperature must be higher than 1500.degree. C. Therefore, as the conductor material, a high melting point metal such as W or Mo must be used. But these metals are higher in conductor resistance as compared with Au, Ag or Cu, and they cannot be soldered. As a countermeasure, hence, it was necessary to coat the surface with Ni and Au.
As one of the methods to solve these problems, researches have been made into the types of dielectric paste suitable for low temperature firing by mixing glass with ceramics such as alumina and magnesia as disclosed in Japanese Laid-Open Patent Applications No. 54-82700, No. 60-8229, and No. 54-111517. The features of such dielectric paste are as follows. inorganic components in the paste are made of ceramics such as alumina and glass. The glass in the paste is softened at the time of firing to form a continuous amorphous network so that each ceramic particle is surrounded by glass. Accordingly, this structure makes the mechanical strength and heat conductivity superior to those of glass alone. Actually, however it is not easy to exhibit the above features fully. One of the reasons for this is that it is not easy to uniformly mix glass and alumina which differ in particle size and specific gravity. Another reason is that wetability and reactivity which are important for promoting sintering of glass and ceramics are not so good. Wwhen these properties are insufficient, in the method of softening of the glass to form the glass network by firing, lumps of ceramic powder are scattered about among the gaps in the glass network, and the chemical reaction between glass and ceramics does not occur easily. As a result, the insulation layer after firing is porous, so that the mechanical strength, insulation resistance, an break-down voltage of the insulation layers are lowered.